1. Field of the Invention
The present invention relates to a power saving circuit for reducing the standby power and start-up power of an electric apparatus, a switching power supply unit for exercising control using a switching device, and a method for setting a switching frequency of a switching power supply unit.
2. Description of the Related Art
Power saving circuits for reducing the standby power and the like of electrical equipment have been widely used, for example, as switching power supply units. FIG. 6 shows the circuitry of a conventional switching power supply unit. This switching power supply unit includes such components as an AC power source CN1, a first bridge rectifier D1, a main switching device Q1, a transformer T1, and a pulse oscillator circuit IC1. The first bridge rectifier D1 is connected to the AC power source CN1. The transformer T1 has a primary winding and a secondary winding, and functions as a transformer circuit. The pulse oscillator circuit IC1 outputs a switching signal to the main switching device Q1.
One end of the primary winding of the transformer T1 is connected to the drain of the switching device Q1 in series. The other end of the primary winding is connected to a positive DC terminal of the first bridge rectifier D1. The negative DC terminal of the first bridge rectifier D1 is connected to the source of the switching device Q1. A smoothing capacitor C8 is connected between the positive and negative DC terminals of the first bridge rectifier D1, and thus functions as a DC power source by virtue of its smoothing operation.
A second bridge rectifier D2 is also connected to the AC power source CN1. The pulse oscillator circuit IC1 is connected between the DC terminals of this second bridge rectifier D2. Series capacitors C3 and C6 are connected between the respective terminals of the AC power source CN1 and the AC terminals of the second bridge rectifier D2, so that an alternating current is introduced into the second bridge rectifier D2. The resulting current rectified by the second bridge rectifier D2 is supplied to the pulse oscillator circuit IC1 for start-up. Both ends of a tertiary winding of the transformer T1 are connected to the pulse oscillator circuit IC1 so that this transformer T1 also supplies power.
An output terminal OUT of the pulse oscillator circuit IC1 is connected to the gate of the switching device Q1. A current detection terminal ISNF of the pulse oscillator circuit IC1 is connected to the source of the switching device Q1.
This switching power supply unit also includes an internal detection circuit 11 for detecting a pulse signal supplied from the pulse oscillator circuit IC1. This internal detection circuit 11 controls the frequency of the switching signal from the pulse oscillator circuit IC1. This internal detection circuit 11 includes a pulse detection circuit 12 for detecting the switching signal (pulse output) supplied from the pulse oscillator circuit IC1. In this instance, resistors R16, R17, and R18, and a capacitor C7 are used for that purpose. This pulse detection circuit 12 is connected between the gate of the switching device Q1 and the output terminal OUT of the pulse oscillator circuit IC1, and outputs the detected switching signal to the base of a switching device Q4.
The internal detection circuit 11 also includes a DC signal level conversion circuit 13. This DC signal level conversion circuit 13 is connected between the collector and the emitter of the switching device Q4. This DC signal level conversion circuit 13 is composed of a plurality of NOT circuits IC4A to IC4D, a diode D7, a capacitor C16, and resistors R23 and R27. Through these components, the pulse signal detected by the pulse detection circuit 12 is converted into a DC signal level. If the pulse oscillator circuit IC1 has a relatively long ON time, the capacitor C16 becomes fully electrically charged. In the meantime, the DC signal level becomes high. If the pulse oscillator circuit IC1 has a relatively short ON time, the capacitor C16 does not become fully electrically charged, in which case the DC signal level becomes low. When the DC signal level input to the NOT circuit IC4B of this DC signal level conversion circuit 13 is low, it is turned into a low frequency instruction signal (in this instance, an ON signal). When the DC signal level input is high, it is turned into a high frequency instruction signal (in this instance, an OFF signal).
The internal detection circuit 11 includes a frequency switching circuit 14. This frequency switching circuit 14 has a switching device Q3. The switching device Q3 has a control terminal which is connected to the NOT circuit IC4B of the DC signal level conversion circuit 13. The low frequency instruction signal (ON signal) or the high frequency instruction signal (OFF signal) output from the NOT circuit IC4B is input to the switching device Q3. The input terminal of the switching device Q3 is connected to a first capacitor C2 in series. A second capacitor C17 is connected in parallel with this series circuit consisting of the switching device Q3 and the first capacitor C2.
Consequently, when the low frequency instruction signal (ON signal) is transmitted to the frequency switching circuit 14, the switching device Q3 turns ON. Since the switching device Q3 is ON, the capacitor C2 is charged in order to lower the switching frequency of the pulse oscillator circuit IC1. Conversely, when the high frequency instruction signal (OFF signal) is transmitted to the frequency switching circuit 14, the switching device Q3 turns OFF. Since the switching device Q3 is OFF, the capacitor C2 is discharged in order to increase the switching frequency of the pulse oscillator circuit IC1 for higher power. (For example, see Japanese Patent Application Laid-Open Nos. 2004-187479 and 2005-151659.)
As shown in FIG. 7, the conventional switching power supply unit has a short time constant when starting-up since components such as the capacitor C2 of the frequency switching circuit 14 will not be charged. Thus, the switching signal of the pulse oscillator circuit IC1 always starts with a high frequency H. The high frequency H accelerates the rise in voltage upon start-up. Nevertheless, an overcurrent (surge current) subsequently flows into the pulse oscillator circuit IC1 through the resistor R5, and the circuit IC1 forcefully switches the frequency to a low frequency L for output suppression. Consequently, a problem has existed in that switching from the high frequency H to the low frequency L occurs even under light load, causing a drop X1 in output. Starting with the high frequency H has also resulted in the problem of increased power consumption.
Moreover, since the frequency switching circuit 14 is configured to use the capacitors C17 and C2 for frequency switching, there has been a problem of poor response, which is ascribable to the charging and discharging operations. Specifically, as shown in FIG. 7, the DC signal level rises while the switching signal has the low frequency L. Then, even if the high frequency instruction signal is input to the frequency switching circuit 14, the frequency-switching instruction can actually be transmitted to the pulse oscillator circuit IC1 only with a delay in timing since the capacitor C2 requires time to discharge. As a result, the output voltage becomes high enough before the switching frequency is switched from the low frequency L to the high frequency H. This causes a large drop X2 in the output capacity upon switching, with the problem that smooth output characteristics cannot be obtained.
There has conventionally been another problem in that the second bridge rectifier D2 is composed of four diodes, i.e., with a high parts count and a large circuit size. Introducing the alternating current into this second bridge rectifier D2 requires two series capacitors C3 and C6, which also contributes to the problem of an increased parts count. There has also been the problem that these components consume power.
Furthermore, according to this circuit, the switching signal of the pulse oscillator circuit IC1 starts with the high frequency before converging to an intended frequency. There has thus been the problem of increased power consumption in standby mode. More specifically, the capacitor C7 is yet to be charged during start-up, and the pulse detection circuit 12 thus has a small time constant and starts with a high frequency. Since this circuit is designed so that the pulse oscillator circuit IC1 repeats the ON (start-up) and OFF (stop) operations at regular intervals when in standby mode (power saving mode), starting with a high frequency upon each start-up has produced the problem of increased power consumption.